Safe disposal of hazardous waste is an increasing societal problem. Once a hazardous waste is generated, disposal and treatment of the waste is heavily regulated by the United States Environmental Protection Agency because of the potentially serious consequences of improper disposal or treatment.
Foundries typically generate large quantities of solid waste which is collected as baghouse dust. These wastes often contain leachable heavy metals, such as lead, cadmium and zinc. If the levels of leachable heavy metals are high, the wastes are classified as hazardous wastes by U.S. EPA or the state regulatory agencies.
Disposal of hazardous wastes containing high levels of leachable heavy metals, such as lead or cadmium, is expensive. In addition to the business cost of completing paperwork associated with the regulation of hazardous waste, tipping fees are usually more than $150 per ton. Transportation costs are also high since hazardous waste must often be transported some distance for disposal in approved facilities. These costs will undoubtedly increase with the U.S. EPA prohibition on land disposal of untreated hazardous waste beginning on May 8, 1990. Therefore, a method of treating hazardous waste to render it nonhazardous could result in tremendous cost savings for generators of hazardous waste, such as foundries.
Solid wastes are classified as hazardous by U.S. EPA for a number of reasons. Certain wastes are classified as hazardous because they contain chemicals which are listed by U.S. EPA as hazardous. Other wastes are classified as hazardous because of characteristics of the waste. These characteristics include ignitability, corrosivity, reactivity, and Extraction Procedure (EP) Toxicity.
Extraction Procedure Toxicity is determined using the Extraction Procedure Toxicity Test (EP Toxicity Test) contained in 40 C.F.R. Part 261, Appendix II, (1989), the disclosure of which is hereby incorporated by reference. In the near future, the Toxicity Characteristic Leaching Procedure (TCLP) will replace the EP Toxicity Test. The EP Toxicity Test and TCLP determine whether a solid waste has unacceptable levels of hazardous substances, such as heavy metals, which can be leached from the waste by infiltrating water. Wastes containing leachable lead and cadmium are currently classified as EP Toxic Waste if the level of cadmium extracted in an EP Toxicity test is above 1.0 mg/L or if the level of lead extracted is above 5.0 mg/L. The upcoming ban on disposal of untreated hazardous waste will require that wastes that are hazardous for lead and/or cadmium leaching not be land disposed without treatment. Some states, notably Michigan, also classify wastes which leach high levels of zinc as hazardous.
The EP Toxicity Test and TCLP are designed to simulate a worst-case leaching situation. These leaching conditions would typically be found in the interior of an actively degrading municipal landfill. In such landfills, the leaching medium is slightly acidic, with a pH of about 5. Additionally, the leaching medium is heavily buffered by volatile organic acids (e.g., acetic acid) produced by the facultative anaerobic degradation of organic matter.
In fact, many industrial wastes are not disposed of under acidic conditions. The actual leaching conditions experienced by these wastes in an industrial landfill may be quite different from the worst-case situation simulated by the EP Toxicity and TCLP tests. In general, high volume hazardous waste, such as foundry baghouse dust, is disposed of in a situation where it is not exposed to a heavily buffered acidic leachate, but rather to unbuffered waters such as rain.
Tests are available which simulate the more typical disposal situation for hazardous wastes such as foundry waste. These tests utilize a relatively unbuffered solution, e.g., deionized water, to provide a better simulation of leaching as it actually occurs in the environment. Examples of nonacidic or not heavily buffered acidic leach tests, commonly referred to as water leach tests, include the Indiana Water Leach Test, which is also called the EP Water Test (Indiana Administrative Code Title 329, Article 2, Solid Waste Management Rule 9); the U.S. EPA Multiple Extraction Procedure (U.S. EPA, 1986, Test Methods for Evaluating Solid Waste, Volume 1C, Method 1320); the American Society of Testing Materials Test (ATSM Standards, method D3987-85); the American Foundrymans Society Leach Test (Ham, R. K., W. C. Boyle and T. P. Kunes, J. Env. Eng. Div. Amer. Soc. Civil Eng., 107 EEL, pp. 155-170, 1981); and the University of Wisconsin SLT Test (Ham, R. K., M. A. Anderson, R. Stegmann and R. R. Stanforth, EPA 600/2-79/109, 1979).
For wastes that are not disposed of in acidic environments, two separate leach tests need to be run to determine whether the waste is hazardous according to regulatory standards, and to determine whether the waste could pose an actual environmental risk when exposed to nonacidic leachate in a disposal facility. The EP Toxicity Test or TCLP test will define the regulatory status of the waste. A water leach test will provide an indication of the actual leaching potential of the waste in the environment.
It is important to utilize both types of tests because lead and zinc are amphoteric metals. Therefore, at alkaline pH values, lead will solubilize and may leach from waste at concentrations that exceed the drinking water standards (maximum contaminant levels) established under the Safe Drinking Water Act. Lead levels may even exceed the hazardous waste standards at alkaline pH values. Because zinc is also an amphoteric metal, it would be expected that zinc could also leach from waste at alkaline PH values, at levels of environmental concern. It should be noted that cadmium is not an amphoteric metal except at extremely high pH and leaching of cadmium from treated waste under nonacidic leaching conditions has not been observed.
These considerations are important because one method that has been used for treating hazardous wastes containing leachable metals is the addition of alkaline materials to control the pH of the EP Toxicity test so that the metals will not be extracted at levels above the hazardous waste standards when the waste is analyzed using the EP Toxicity Test or TCLP test. A number of alkaline additives have been used which chemically react with metals in the waste and control the leaching test pH. (See, for example, Hickock, E. A., and Associates. 1984, "Foundry Waste Stabilization: Laboratory Testing and Conceptual Equipment Design", Report to Participating Foundry Groups, 47 pages; California Cast Metals Association, 1989, "Detoxifying Foundry Sand", Report to Members; Nagle, D.L., R. R. Stanforth, P. E. Duranceau and T. P. Kunes, 1983, AFS Transactions, Vol. 91, pp. 715-720; Turpin, P. D., T. R. Stolzenburg, W. A. Stephens, and T. P. Kunes, 1985, AFS Transactions, Vol. 93, pp. 737-740; and U.S. Pat. No. 4,889,640 issued Dec. 26, 1989 to Stanforth).
Hazardous wastes containing leachable metals that are treated with alkaline additives currently meet regulatory standards for land disposal. However, the tests used for regulatory purposes, the EP Toxicity Test and TCLP, do not necessarily simulate actual environmental conditions. Consequently, it is possible to treat a waste with chemical additives to reduce leaching of metals such as lead and cadmium in the EP Toxicity Test and TCLP to render it nonhazardous from a regulatory standpoint, while at the same time increasing the pH to the point where lead is solubilized when a water leach test is run. For wastes containing zinc, it would also be expected that zinc could leach at levels of environmental concern. Since the water leach test simulates common disposal conditions in the environment, this indicates that it is possible to create an environmental problem in the process of solving a regulatory problem.
The data in Table 1 show the problem with present treatment methods which reduce leaching of lead and cadmium when a treated waste is analyzed using an acidic leach test such as the EP Toxicity Test or TCLP, but simultaneously increase leaching of lead when treated waste is analyzed using a water leach test. Note that some of the additives, particularly cement, are commonly thought to chemically bind up heavy metals (U.S. EPA (1989), "Stabilization of CERCLA and RCRA Wastes", publication EPA 1625/6-89/022, Cincinnati, Ohio). As shown in Table 1, the binding was not sufficient to immobilize the metals at alkaline pH values.
TABLE 1 ______________________________________ TREATMENT OF EP TOXIC WASTES WITH ALKALINE ADDITIVES: EP TOXICITY AND WATER LEACH TEST RESULTS EP Toxicity Test EP Water Test Final Cd Pb Final Cd Pb Sample pH mg/L pH mg/L ______________________________________ 1. 5.6 8.8 79 Not Tested Untreated Treated with Cement +20% 7.2 3.0 12 11.5 &lt;0.005 12 +30% 9.4 &lt;0.005 0.3 11.9 &lt;0.005 26 2. 5.0 9.8 85.1 9.8 0.0034 0.022 Untreated Treated with 9.7 0.029 0.3 10.8 0.0003 1.3 Magnesium Oxide +7.5% 3. 5.1 9.2 9.7 6.5 6.8 3.4 Untreated Treated with Calcium Oxide +4% 6.9 -- 11.0 11.9 -- 16.0 +6% 8.9 0.23 0.3 12.1 &lt;0.01 5.5 +8% 9.7 0.04 0.4 12.4 &lt;0.01 8.8 +10% 10.0 0.02 0.3 12.4 &lt;0.01 8.6 +12% 11.0 0.01 2.4 12.5 &lt;0.01 8.6 EP Toxicity 1.0 5.0 Test Standards Drinking Water 0.01 0.05 Standards (MCLs) ______________________________________
There are single compound additives that can be used to control leaching in both an EP Toxicity Test and a water leach test, that are not covered by this invention disclosure, for example, reactive calcium carbonate and reactive magnesium carbonate as disclosed in U.S. Pat. No. 4,889,640, which is incorporated herein by reference. However, a problem with reactive calcium carbonate is that it is less effective at preventing the leaching of lead and cadmium, if the solid waste contains high levels of zinc, because zinc interferes with the conversion of lead and cadmium to carbonate salts.